1. Field of the Invention
This invention relates to an interline transfer or frame interline transfer CCD type solid state image sensor. More particularly, it relates to a CCD type solid state image sensor having an improved horizontal charge transfer section.
2. Description of the Prior Art
With a solid-state image device having an extremely large number of pixels, transfer of signal charges becomes difficult because of the increased horizontal transfer frequency. For lowering the horizontal transfer frequency, it has been envisaged to provide two or more horizontal charge transfer sections or horizontal registers and to transfer signal charges by distributing the signal charges thereto.
For example, in a solid state image sensor shown in FIG. 1 in which there are provided two parallel horizontal charge transfer sections 1 and 2 and in which signal charges are transferred at an elevated speed through the horizontal charge transfer sections by two-phase transfer signals .phi.H1 and .phi.H2, a transfer electrodes 3 of the two horizontal charge transfer sections, striding over transfer gate 4, are used in common, and are supplied with common transfer signals .phi.H1 and .phi.H2. A channel region 5, sandwitched between channel stop regions 6, is provided below the transfer gate 4, and signal charges are transferred from the horizontal charge transfer section 1 to the horizontal charge transfer section 2 by way of the channel region 5. The channel region 5 is sandwitched between a region of the horizontal charge transfer section 1 supplied with the transfer signal .phi.H2 and a region of the horizontal charge transfer section 2 supplied with the transfer signal .phi.H1. The region 7 of the horizontal charge transfer section 1, supplied with the transfer signal .phi.H1, is contiguous to the channel stop region 6 below the transfer gate 4.
FIGS. 2a to 2d are diagrams for illustrating charge transfer between horizontal charge transfer sections from the aspect of the electrical potential. It is assumed that signal charges are transferred from a region of the transfer signal .phi.H2 of the horizontal charge transfer section 1 to a region of the transfer signal .phi.H1 of the horizontal charge transfer section 2 by way of transfer gate 4 controlled by a signal .phi.HHG. First, as shown in FIG. 2a, the signal HHG is raised in level to render the transfer gate 4 conductive, while the transfer signals .phi.H1 and .phi.H2 are set to a low level. This causes charges to be stored in the channel region 5 below the transfer gate 4. Then, as shown in FIG. 2b, only the transfer signal .phi.H1 supplied to the destination is changed to a higher level. This causes the charges stored in the channel region 5 below the transfer gate 4 to be transferred to a region supplied with the transfer signal .phi.H1 of the other horizontal charge transfer section. Then, for turning the transfer gate 4 off, the signal .phi.HHG is changed from the high level to the low level, as shown in FIG. 2c. When the signal .phi.HHG is completely in the low level, as shown in FIG. 2d, the two horizontal charge transfer sections 1 and 2 are electrically isolated from each other to complete the distributed transfer of the signal charges.
The above described charge transfer between the horizontal charge transfer sections has the following demerit.
That is, while the transfer gate between two horizontal charge transfer sections need to be turned off on termination of transfer of signal charges, the signal charges present in the channel region 5 below the transfer gate when the level is changed to turn off the transfer gate tend to be returned to the horizontal charge transfer section 1 to lower the transfer efficiency.
The second demerit is the smear which is produced in the solid-state imaging device, such as the CCD imager, due to the excess quantity of the incident light. As a means for eliminating the smear, there is known a technique of providing a drain region for sweeping out unnecessary charges at the proximal end in the forward transfer direction of the vertical register of the imaging section to effect reverse transfer of signal charges, as shown for example in "The Journal of The Institute of Television Engineerings of Japan, Image Information Engineering and Broadcasting Technology", vol. 141, No. 11, 1987, pages 1039 to 1046, or a technique of transferring signal charges in the forward direction through the horizontal register to sweep out unnecessary signal charges to a precharge drain at the terminal end of the horizontal register.
Meanwhile, with the technique of reverse transfer for sweeping out unnecessary signal charges, it becomes necessary to provide a circuit for transferring the charges in the reverse direction, so that a complex operation is necessitated to control the circuit. On the other hand, with the technique of sweeping out smear charges by the transfer through the horizontal register if an overflow occurs in the horizontal register by smear charges, it may present itself as an image defect.
Considering a device having a drain region for sweeping out unnecessary charges, which is provided along a horizontal register in adjacency to the side of the horizontal register opposite to the imaging section, it is necessary to lower the resistance of the drain region for efficient sweepout of the unnecessary charges. With such device, since a drain region is provided between the busline wiring and the horizontal register, a wider width of the drain region leads to an increased area on the chip and to an increased length of the transfer electrode of the horizontal register, thereby causing the propagation delay of the transfer clocks to the transfer electrode.